The invention relates to a method for building up a phosphate deposit in which method a phosphate containing aqueous liquid is passed from an input through a reaction space at least laterally defined by a layer having poor hydraulic conductivity and in which method said reaction space is activated by means of the root system formed by plants of Limnophytae species. It is well known that every year large amounts of soluble fertilizer phosphates of inorganic or of organic origin become irreversibly bound to agriculturally used soil and thus become removed from resorption by plants or any other potential utilization. Numerous reactions participate in the overall binding process and are discussed extensively in the professional literature. In agriculturally used soils these processes pertain just to the upper soil layer which is penetrated by the roots of the respective plants and which regularly does not comprise depths of more than 30 to 35 cm of the soil profile.
According to the long term view phosphate fixation in the soil matrix occurs through sorption, ligand exchange with clay minerals, synthesis of organic phosphates by micro-organisms, formation of apatites in the form of precipitated bodies and will result in the build-up of iron phosphates (Vivianite and Strengite) and aluminum phosphate (Variscite) depending on
(a) the presence in the soil of reactive iron and aluminum oxides and hydroxides and
(b) the realization of their reaction with dissolved ortho-phosphate.
It is known for agriculturally used soils in our latitudes that their average iron and aluminum oxide contents combined with biological and non-biological processes occurring in the soils may yield an annular phosphate fixation of about 75 kg over an area of 1 hectare. This quantity, together with determinants for the hydraulic properties of the soil, represents a significant number for a real sewage clarification by means of "soil treatment" since this number will limit the waste removal capacity of the soil to 100 population equivalents per hectare area.
The aforementioned process cannot result in genesis of economically considerable phosphate deposits as will be evident from the following balance:
______________________________________ fixation rate, per annum and per hectare 75 kg P reactive space in the soil matrix participating, per hectare about 3000 m.sup.3 population equivalents, per hectare about 100 fixation capacity at an iron content of 5 percent and at pH 7, tons per hectare about 55 fixation capacity in terms of time, years about 750 index value of phosphate deposited after 100 years, based on the current price of US-$ 1,700 per ton of phosphate about US-$ 13,000 index value of phosphate deposited after 750 years at exhaustion of capacity about US-$ 100,000 ______________________________________
In this balance the index value of the phosphate deposit or its richness, respectively, as built up after 100 years is of interest since within about this span of time a crisis condition will develop with respect to the present state of phosphate supply. The richness of phosphate deposit accumulated within this term will not exceed 7.5 tons per hectare.
It is also known that in the rhizosphere sewage treatment method as mentioned initially (see R. Kickuth, "Utilization of Manure by Land spreading" in EEC Commission EUR 5672e, Janssen Services, London, 1977, pages 335 to 343) considerable amounts of phosphate become eliminated. Therein, Limnophytae of the species Phragmites, Juncus, Typha, Schoenoplectus or Eleocharis are planted in the reactive space which by means of a rather voluminous aerenchymatic tissue not only supply the roots as such but also the adherent rhizosphere with oxygen and thus very effectively eliminate organic and nitrogen containing pollutants from the water. The reactive space has a depth of about 60 cm and is passed through horizontally from an input by the water to be purified.
All the methods for treating sewage water, sewage sludge and waste known today do not lead to neogenesis of useful phosphate deposits.
Phosphates are associated with the group of raw materials the supply of which will become exhausted particularly rapidly. While estimates of the supply accessible using presently available means differ considerably, even the most favourable prognoses yield a time span of only 150 years, the most unfavourable ones, however, just 50 years until the known and presumed workable deposits will have become exhausted.
In consideration thereof the annular world-wide introduction of tremendous amounts of phosphates into life supporting and manufacturing processes and, as a consequence thereof, surrendering the same as waste, particularly as sewage components, and additionally as a waste burdening the environment to seawardly directed water flows or deposing the same as sludge or waste in extreme dilution, cannot be advocated anymore.
It is the object of the invention to provide for a method of the initially mentioned kind which is suited for the build-up of a phosphate deposit.
In accordance with the invention this object is achieved by planting the Limnophytae in a reactive space including a soil matrix containing iron and/or aluminum. Thereby it is achieved that the rate and the extent of phosphate fixation in the soil matrix are increased by a big step using correspondingly aimed technical and biotechnical measures so that phosphate supplies are established thereby which may become economically exploited.
Conveniently, the Limnophytae for carrying out the method according to the invention are selected from those species which are adapted to supply oxygen from the air via their aerenchymatic tissue to the root system and, additionally, to the adherent rhizosphere and to thereby form in the binding hydromorphous soil matrix a pattern of aerobic and anaerobic micro compartments. To accelerate the reaction between the phosphate and the iron and/or aluminum compounds those species of Limnophytae are selected for the method according to the invention which by their primary or secondary metabolism give off substances into the reactive space which will bind iron and/or aluminum to form water soluble complexes of high formation constants.
For processing phosphate containing aqueous liquids including organic and inorganic burdens such species will be selected from the Limnophytae which will degrade the burdens by means of organisms associated with the rhizospheres of the Limnophytae.
Particularly suited and active partners in the system are plants of the associated reed-type plants which due to their specific morphological and physiological features determine the turnover of material in the soil matrix to a high degree. In particular the species Phragmites, Typha, Carex, Glyceria, Eleocharis, Juncus, Iris pseudacorus, Iris versicolor, Schoenoplectus, Phalaris, Arundo are associated therewith.
In the method according to the invention the reactive space comprises a soil matrix containing fine and coarse clay in the range of 30 to 70 percent by weight based on the total weight of the dry soil matrix and an expandable clay mineral forming a three-layered structure in the range of 10 to 20 percent by weight related to the clay contents. The soil matrix may comprise an iron and/or aluminum rich material which may be selected from the group: spatic iron ore, laterite, terra rossa, bog ore, pea ore, iron ore roast residues, and oolithic and breccious sediments from iron ore mining operations. The iron and/or aluminum rich material also may become dispersed in the phosphate containing aqueous liquid.
Depending on respective conditions of inclination and of hydraulic conductivity the phosphate containing aqueous liquid is introduced into the reactive space along one side thereof either at the surface or below thereof again in dependence on respective local conditions.
In accordance with the invention, therefore, phosphate fixation in the soil matrix is increased by a significant step resulting in economically useful phosphate deposits. The measures employed therefor are directed to
1. affecting soil conductivity in such a way as to render the same receptive to phosphate containing liquids,
2. increasing the depth of the reactive space to about 1.20 m,
3. providing a matrix pattern comprising aerobic and anaerobic micro compartments, and
4. accelerating the chemical reaction of the phosphate present in the liquid with the iron and/or aluminum available from the soil matrix.